Uner the same assumption, I prefer a regular expression. The Common module will match any valid number. The /e option will do the subtraction. The /r option returns the result without modifying the original string.

use strict;
use warnings;
use Regexp::Common;
my $NUMBER = qr/$RE{num}{real}/;
my @array = ('31:15');
print $array[0] =~ s/($NUMBER):($NUMBER)/$1-$2/er;
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Regexp::Common

While nice and general, this seems a bit heavy for the OP, especially considering the the rather regular examples given. Of course, always nice to see an example of a more general/powerful solution.