lc is "lowercase". I'm not sure about the comma.
It's there because of this: 
$ cat foo.pl
my %a;
$_='fOo';

$a{lc}++;
$a{lc()}++;
$a{+lc}++;
$a{lc,}++;

$ perl -MO=Deparse foo.pl
my %a;
$_ = 'fOo';
++$a{'lc'};
++$a{lc $_};
++$a{lc $_};
++$a{lc $_};
foo.pl syntax OK

[download]
So it's there for disambiguating too, just as +shift. I chose the latter form because in this context (involving the ++ operator) it seemed to me to be the least obtrusive and visually distracting.

In reply to Re^3: Common elements of a hash of hashes by blazar
in thread Common elements of a hash of hashes by Anonymous Monk

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