I want to sort a hash's %h keys in an arbitrary order @seq. I can achieve that by building a helper hash %seq_helper in which the first key is the first item in @seq and the first value is 0, the second key is the next item in @seq and the second value is 1 and so on. Then I can easily compare the items of @seq using <=> and the values of %seq_helper.

However, it does not seem to be possible to make this into a sorting subroutine that accepts a list that tells the sub how I want the items to be sorted. The following is a syntax error after sort by_seq. 

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

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➀ Why?

➁ How would you solve it?

In reply to sort in arbitrary order with subroutine, syntax error by Anonymous Monk

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