Very nice! This requires perl 5.8.0 and above. When I ran it with perl 5.6.1 and perl 5.005 I got the error message:

Invalid type in unpack: '(' at g.pl line 3.
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Update: from perl58delta: "pack() / unpack() can now group template letters with () and then apply repetition/count modifiers on the groups". Also, I think you need to reverse the reverse and scalar. Running:

use strict;
sub commify {
    reverse scalar join ',',
    unpack '(A3)*',
    reverse scalar shift;
}
sub commify2 {
    scalar reverse join ',',
    unpack '(A3)*',
    reverse scalar shift;
}
my $x = commify(1234);
my $y = commify2(1234);
print "x='$x' y='$y'\n";
print "comm :", commify(1234), ":\n";
print "comm2:", commify2(1234), ":\n";
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produces:

x='1,234' y='1,234'
comm :432,1:
comm2:1,234:
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