Please be advised that the norm is to include, if not the full, then a compilable/working sub-set of the, code that, when run, manifests the problem(s) you encountered on the way. Having said that...

use warnings;
use strict;

use Data::Dumper;

my @files = <DATA>;
chomp @files;

warn Dumper \@files;

sub get_least($) {
    my $md5 = shift;

    my $num = (
        sort map { s/${md5}_//; s/_max//; ($_) } grep /^$md5.*_max/, @
+files
    )[0];

    warn("No max file"), return undef unless $num;
    return $num;
}

warn get_least(q/111aaa222ccc324567fed54333221235/);
warn get_least(q/777aaa222ccc324567fed54333221235/);
warn get_least(q/99999999999999999999999999999999/);

__DATA__
111aaa222ccc324567fed54333221235_1.04
111aaa222ccc324567fed54333221235_1.05_max
111aaa222ccc324567fed54333221235_0.98_min
111aaa222ccc324567fed54333221235_1.02_max
111aaa222ccc324567fed54333221235_0.21
777aaa222ccc324567fed54333221235_1.04
777aaa222ccc324567fed54333221235_1.07_min
777aaa222ccc324567fed54333221235_1.04_max
99999999999999999999999999999999_1.04
99999999999999999999999999999999_1.07_min
99999999999999999999999999999999_1.04_min
[download]

when run, gives

$ perl tst.pl
$VAR1 = [
          '111aaa222ccc324567fed54333221235_1.04',
          '111aaa222ccc324567fed54333221235_1.05_max',
          '111aaa222ccc324567fed54333221235_0.98_min',
          '111aaa222ccc324567fed54333221235_1.02_max',
          '111aaa222ccc324567fed54333221235_0.21',
          '777aaa222ccc324567fed54333221235_1.04',
          '777aaa222ccc324567fed54333221235_1.07_min',
          '777aaa222ccc324567fed54333221235_1.04_max',
          '99999999999999999999999999999999_1.04',
          '99999999999999999999999999999999_1.07_min',
          '99999999999999999999999999999999_1.04_min'
        ];
1.02 at tst.pl line 22, <DATA> line 11.
1.04 at tst.pl line 23, <DATA> line 11.
No max file at tst.pl line 18, <DATA> line 11.
Use of uninitialized value in warn at tst.pl line 24, <DATA> line 11.
Warning: something's wrong at tst.pl line 24, <DATA> line 11.
[download]

which, I believe, is as required.