I'm not quite sure from your post exactly what it is you want to do. I'm guessing that there are multiple "interface" descriptions in the configuration file and you need to extract one key/value pair from each. Processing line by line is probably not a good idea as you could get an interface with a good description line but a bad switchport line followed by the converse, thus ending up with a key from the one associated with a value from the other.

As long as the configuration file was of manageable size I would read the whole file into a string and then split it into individual interfaces using a look-ahead pattern. Then I would iterate over the interfaces extracting both key and value using a single regex and adding them to the hash.

use strict;
use warnings;

use Data::Dumper;

my $data = do
   {
       local $/;
       <DATA>;
   };

my @interfaces = split m{(?=interface)}, $data;

my $rxExtract = qr
   {(?xms)
       description\s+
       (\S+)
       .*?
       switchport\strunk\sallowed\svlan\s*
       ([^\n]+)
   };

my %hash;

foreach my $interface ( @interfaces )
{
    $hash{$1} = $2 if $interface =~ $rxExtract
}

print Data::Dumper->Dumpxs([\%hash], [q{*hash}]);

__END__
interface Port-channel31
 description abc.xyz-conf
 no ip address
 load-interval 60
 switchport
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 15-17,30,35
interface Port-channel32
 description def.uvw-conf
 no ip address
 load-interval 55
 switchport
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 12-15,31-38
interface Port-channel35
 description ghi.rst-conf
 no ip address
 load-interval 75
 switchport
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 21-26,44,47,51
[download]

Here's the Data::Dumper output.

%hash = (
          'ghi.rst-conf' => '21-26,44,47,51',
          'def.uvw-conf' => '12-15,31-38',
          'abc.xyz-conf' => '15-17,30,35'
        );
[download]

As I said, I'm not sure if this is what you want but, hopefully, it will give you some help.

Cheers,

JohnGG