#!/usr/bin/perl

use strict;
use warnings;

my @array = qw(a_ a_b_ p_ p.x_);
@array = sort{ length $b <=> length $a }@array;
my $rx_string = join '|', map{ qr/\Q$_\E/ }@array;
my $regex = qr/^($rx_string)/;

for my $string ( qw(a_b_file a_test) ) {
    if ( $string =~ $regex ) {
        print $1,"\n";
    }
}