Another version:

use warnings;
use strict;

my @input = (-1, 1, 2, 3, 5, 6, 8, 10, 11, 12, 13 );
my @ranges = ( [ $input[0] ] );
for (@input[1..$#input]) {
    if( $_ == $ranges[-1][-1]+1 ) {
        $ranges[-1][1] = $_;
    } else {
        push @ranges, [ $_ ];
    }
}
$" = ":";
print "@$_\n" for @ranges;
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Update: replaced "eq" with "==" above.