use strict;
use warnings;
use 5.010;

my @filenames =('XYZ12345_X05_20110805_9999999.TXT', 'XYZ12345_X05_20110805_9999999_00001.TXT');
for my $filename (@filenames) {
    my $rd = int( rand(100000)) + 999999;
    say "unchanged_filename: $filename";
    $filename =~ s/_00001\./_$rd./;
    say "changed_filename: $filename";
}