sub show {
    my $line = $_;
    ...
}
[download]

Arguments to routines are passed in the array @_, not $_. So either write

sub show {
    my $line = shift;
[download]

or

sub show {
    my ($line) = @_;
[download]

or

sub show {
    my $line = $_[0];
[download]

With that change, things should work just fine, provided the loop that reads from the socket is in the main/parent process — which it seems to be.