#!/usr/bin/perl --
use strict;
use warnings;
use Data::Dump qw/ dd /;
my @in = qw/
1020300000_XXXXXXXXX_20160707193000.TXT
1020300000_XXXXXXXXX_20160707170000.TXT
1020300000_XXXXXXXXX_20160707190000.TXT
/;
my $final = GiveMeItTheThingIWant( \@in );
dd( $final, \@in );
sub GiveMeItTheThingIWant {
    my( $in ) = @_;
    my @ordered = 
    map { $$_[1] }
    sort { $$a[0] cmp $$b[0] }
    map { 
#~         my( $left, $right ) = $_ =~ m{^([^_]+)_[^_]+_([^_]+)};
        my( $left, $middle, $right , $txt ) = split /[_\.]/, $_;
        [ $right, $_ ];
    }
    @{$in};
    
    return $ordered[0];
#~     return '1020300000_XXXXXXXXX_20160707170000.TXT';
}
__END__