use strict;
use diagnostics;

sub by_seq {
    my $seq_aref = shift;
    my %seq_helper;
    {
        my $index = 0;
        for my $item (@{ $seq_aref }) {
            $seq_helper{$item} = $index;
            $index++;
        };
    };

    return $seq_helper{$a} <=> $seq_helper{$b};
};

# --

my %h = (one=>2,three=>4,five=>6,seven=>8,nine=>0);

for (sort by_seq([qw(five nine one seven three)]) keys %h) {
    print "$_: $h{$_}\n";
};
__DATA__
expected output
five: 6
nine: 0
one: 2
seven: 8
three: 4