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;