When computing with integer ranges, Set::IntSpan is a useful module:

use Set::IntSpan;
use warnings;
use strict;

my $set_spec1 = '21-45';
my $set_spec2 = '44-60';

my $set = new Set::IntSpan $set_spec1;
my $u_set = union $set $set_spec2;
my $runlist = run_list $u_set ;
print $runlist;
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This prints 21-60.

Now, computiing integer ranges from your times (60*hour+ minute), you can combine ranges as above.