Besides, the "eq" is actually converting the integer back to a string, which will cost performance.

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

use Benchmark 'timethis';

timethis (10_000_000, "valid_int ('18446744073709551614')");

sub valid_int {
    my $num = shift;
    return unless $num =~ /^\d+$/a;
    return int $num eq $num;
}

__END__

timethis 10000000:  8 wallclock secs ( 7.93 usr +  0.00 sys =  7.93 CP
+U) @ 1261034.05/s (n=10000000)
[download]

So, less than a microsecond on my aging system. Doubtless this can be improved upon (although about half the time taken appears to be the regex so there's a limit there too).