Apart from the suggestions already made, you can just do the calculation of years and months if you're just interested in "whole" month numbers, and bypass the issue of what is a month altogether:

sub months {
    my (undef,undef,undef,$day,$month,$year) = localtime;
    $_[0] =~ m#^(\d+)/(\d+)/(\d+)$#;
    (12 * (1900 + $year - $1)) + 1 + $month - $2 - ($day < $3);
}
print months( "1999/06/30" ),$/;
print months( "2004/01/24" ),$/;
print months( "2004/01/25" ),$/;
__END__
55
1
0
[download]

Please note that I don't think "56" is the correct answer for today. It's still 6 days from the 30th, and only then we'd be 56 months from the given date. Check the other examples.

Of course, all of this doesn't take into account anything like timezones. But if you're only interested in whole month granularity, I think that's pretty ok.

Liz