#!/usr/bin/perl

use strict;
use warnings;

my $filedata = "Here is XYZ_ABC_123456.7890 Foo Bar";

my $mode = ( @ARGV and $ARGV[0] =~ /u/ ) ? ":utf8" : '';

open I, "<$mode", \$filedata ;
while (<I>) {
    my ( $id ) = ( /(XYZ_ABC_[\d.]+)/ );
    print "ID is $id\n";
}