The output should be (\d+:\d+) so which is required by the Verilog syntax I had to use.

Does the "Verilog syntax" have some sort of specification that says an N1..N2 range should be presented sometimes as "N1:N2" and other times as "N2:N1"? Do you know whether there is some reason for this fluctuation?

Regarding the code you posted, you are missing some fundamental points of Perl syntax. What do you think  @digits ne '' is supposed to do? (Hint: an array in a scalar context always returns the number of elements in the array, which will never be the same as an empty string.)

If "Verilog syntax" supports the use of a consistent ordering of range components, I think this will be close to what you're looking for:

use strict;
use warnings;

my @test = qw/1 5:10 18:14 21 24:25 31:28/;
print get_reserved( \@test ), "\n";

sub get_reserved
{
    my ( $bits_ref ) = @_;  # should be an array ref
    return "bad call to get_reserved()"
        unless ( ref( $bits_ref ) eq 'ARRAY' and @$bits_ref );

    my @reg32set;
    for my $spec ( @$bits_ref ) {
        if ( $spec =~ /(\d+):(\d+)/) {
            my ( $bgn, $end ) = sort {$a<=>$b} ( $1, $2 );
            $reg32set[$_] = 1 for ( $bgn .. $end );
        }
        else {
            $reg32set[$spec] = 1;
        }
    }

    my $verilog_string = '';
    for my $seq ( 0 .. $#reg32set ) {
        if ( defined( $reg32set[$seq] )) {
            $verilog_string .= ", ";
        } else {
            $verilog_string .= "$seq:";
        }
    }
    return "no intervals to report from get_reserved()"
        unless ( $verilog_string =~ /\d/ );

    $verilog_string =~ s/(?<=\d):[\d:]+:(?=\d)/:/g;  # e.g. "2:3:4:5:6
+ -> 2:6
    $verilog_string =~ s/^(?:, )+//;
    $verilog_string =~ s/:(?:, )*$//;
    $verilog_string =~ s/:,(?: ,)*/,/g;

    return $verilog_string;
}
[download]

(updated to add diagnostic messages for a couple of edge conditions)