For the mechanics of calling POSIX functions, you can keep the overhead down by only importing the names you want to use:

#!/usr/bin/perl
use warnings;
use strict;

use POSIX qw( isdigit );
for (<DATA>) {
    print $_,
          isdigit($_) ? ' is' : ' is not';
          ' all digits.',
          $/;
}
__DATA__    
7
5
A
75A
433
12340
[download]

Note that perl's POSIX isdigit takes strings of any length, so this is not just like /\d/. The equivalent is /^\d+$/.

After Compline,
Zaxo