A number that must be used as a key ..... The first and last column should be the sort keys.

:) you gave sample input and sample output

the sample output does not appear in the sample input

its not clear what you mean by "first column code" and I cannot deduce/guess from the sample you provided

The word explanations do not clarify which part is which

See this example: 1020300000_XXXXXXXXX_20160707193000.TXT 1020300000_XXXXXXXXX_20160707170000.TXT I need to return only the oldest file using the file name date. 1020300000_XXXXXXXXX_20160707170000.TXT

#!/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__
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Ok thats easier to deduce from, its easier to understand the question if you take one file name, break it into parts, and label the parts so they match the word descriptions ... I'll be back in 1min