• Read the file line-by-line, and look for "description" and "switchport" lines, extracting your key and value if you find them.
• Reset key and value if the line is an "interface" line.
• Store the key/value pair in a hash when you have both

use Data::Dumper;

my ( $key, $value, %hash2 );
while ( <DATA> ) {
    $key = $value = undef and next;
        if /^interface /;
    $key   = $1 if /description\s+ ([^\"][^\s]+)/x;
    $value = $1 if /switchport trunk allowed vlan\s*(.*)/;

    next unless $key && $value;
    $hash2{$key} = $value;
    $key = $value = undef;
}

print Dumper \%hash2;
[download]

Using johngg's test data gives:

$VAR1 = {
          'ghi.rst-conf' => '21-26,44,47,51',
          'def.uvw-conf' => '12-15,31-38',
          'abc.xyz-conf' => '15-17,30,35'
        };
[download]