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

##</code><code>##

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