#!/usr/bin/perl -w
use strict;

sub human
{
    my (@r, $g, $s, $l);
    while (@_) {
        $l = $g = $s = shift;
        $l = shift while (++$g eq ($_[0] || ''));
        push(@r, $s eq $l ? $s : "$s-$l");
    }
    @r;
}

print join(", ", human(1..12, 14..21, 'aa'..'af', 'zz'..'aaf')), "\n";
print join(", ", human(1, 2, 3, 5, 7, 8, 9, 11, 14)), "\n";
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Output is:

1-12, 14-21, aa-af, zz-aaf
1-3, 5, 7-9, 11, 14
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update: made work with solo items (thanks CharlesClarkson!)