use strict;
use warnings;
use Data::Dumper;

my @xml_files = map { $_->[0] }
                sort { $a->[1] <=> $b->[1] }
                map { [$_, (split /_/, $_)[1]] }
                <DATA>;

print Dumper \@xml_files;

__DATA__
123_20070303_REFNUM.xml
123_20071225_REFNUM.xml
123_20080101_REFNUM.xml
923_20070227_REFNUM.xml