A clean solution:

sub collapse_seq {
    return () if !@_;

    my @list = sort { $a <=> $b } @_;
    my @seqs = [ (shift(@list)) x 2 ];

    for ( @list ) {
        if ($seqs[-1][1] + 1 == $_ ) {
            $seqs[-1][1] = $_;
        } else {
            push @seqs, [ $_, $_ ];
        }
    }

    return
        map { $_->[0] == $_->[1] ? $->[0] : "$->[0]-$->[1]" }
        @seqs;
}
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