Carrying an object close to the start of Shabbos:
The public area is defined as a Reshus Harabim: It is forbidden for one to carry an object in his hand [or pocket] into a public domain which is defined as a Reshus Harabim very close to the start of Shabbos. This is due to a decree that one may come to forget to rid himself of the object prior to the start of Shabbos, and he will come to transgress the laws of carrying. [Furthermore, this prohibition applies even while one is still in his own house, and thus he may not take hold of an object near the start of Shabbos with intent to exit with that object to a Reshus Harabim.]
The public area is defined as a Karmalis: If the public domain of one’s area is defined as a Karmalis, then there is no prohibition to carry items into it, even in ones hand, very close to Shabbos [even if there is no Eiruv in the area].
Practically are our public areas defined as Reshus Harabim or as a Karmalis? Today the custom has spread to follow the opinion which considers all public areas as a Karmalis, and accordingly there is no longer any prohibition to carry an item very close to Shabbos into the public domain. [see footnote]
It is forbidden to carry an object into a rishus harabim close to the beginning of Shabbos, as this may lead to one carrying it on Shabbos. This prohibition does not apply to carrying into a Karmalis or to carrying something that is only Rabinically forbidden to carry on Shabbos.
Practically today being that all areas are considered a Karmalis, there is no longer a prohibition to carry items close to Shabbos into a public area.
 Mishneh Berurah 252/52; as is also evident from Admur 252/18 regarding a pin stuck in ones clothes that it was only allowed to walk with it outside on Erev Shabbos because it is only a Rabbinical prohibition to carry it on Shabbos.
Accordingly it requires further analysis why Admur wrote that it is specifically forbidden to carry an item in ones hand and does not also write in one’s clothes, as we wrote above. Now, although the Michaber himself wrote “hand” that is because he is dealing with a case of a needle, of which when on ones clothes is allowed. [See Biur Halacha 252 “Beyado”] However Admur mentions generally “an item” and thus why does he use the words “in ones hands” at all, if in truth the same law would apply if the object is on one’s clothes.
 This prohibition however only applies to items which carrying them on Shabbos is Biblically forbidden. However it is permitted to carry close to Shabbos an item which on Shabbos itself is only Rabinically forbidden to carry, such as a needle pinned to ones clothes. As although it is Rabinically forbidden to carry this item on Shabbos itself, the Sages did not decree against doing so near Shabbos due to worry that he may forget and carry it, as this would be a decree made to protect a matter which is itself a decree, of which the Sages do not make. [ibid]
 Such as to hide the object if it is of value. [ibid]
For this reason this prohibition does not apply to Tefillin which one is wearing in a Reshus Harabim close to Shabbos being that one must always be conscious of Tefillin when they are on him and he will certainly remember to remove them prior to the start of Shabbos. [252/19] However if one is not wearing the Tefillin and is merely carrying them, then the above decree applies to it as well. [Mishneh Berurah 252/54 in name of M”A]
 Either by carrying the object into a private domain, or by carrying it four Amos within the Reshus Harabim.
This transgression however is only Biblically applicable if, after Shabbos began, one remembered that he was holding the object and despite this continued to carry it [such as he forgot that Shabbos began]. If however one did not become aware of the object after Shabbos began then he does not transgress a Biblical command by carrying it, as the Torah only forbade doing a Melacha which one is aware of its occurrence, which means that the person intends knowingly to do that Melacha, and it is only that if in such a case he did not remember that it was already Shabbos, or that this Melacha is forbidden that he would be liable to bring a sin offering. [ibid]
 So is implied from Admur which mentions that “he may come to carry it from a private domain into a public domain” hence clearly implying that the person is currently standing in a private domain, and it is nevertheless still forbidden. So rules also Peri Megadim 252 A”A 26, based on the wording of the Michaber.
 As carrying in a Karmalis is itself only a Rabbinical prohibition, and the Sages do not make a decree upon a decree. [ibid]
 As they hold that a true Reshus Harabim is no longer in existence today. [ibid]
 Vetzaruch Iyun if according to Admur there is any need for one to be stringent, if he so chooses, to suspect for the opinion which rules that even today there is a Reshus Harabims. On the one hand if there were no need to be stringent at all, then why did Admur mention the entire lengthy discussion in this matter rather than simply stating that today the prohibition does not apply. On the other hand the Ketzos Hashulchan  completely omits this entire discussion, seemingly learning that there is no longer any need to suspect for this at all today.
Opinion of Mishneh Berurah: The M”B [252/52] concludes that according to many Poskim one is to be stringent.
Requires Analysis: To note from 266/15 that Admur states that one who carried a wallet into Shabbos was negligent that he took it with him. This is not understood in light of the above ruling of Admur that today we hold all areas has the status of a Karmalis, and thus what negligence is there. [Glosses of Tzemach Tzedek on Admur, printed in Shulchan Aruch 2 p. 836].