hangs with an empty pipe how do I fix that?

By only reading from it if it is a pipe...

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1176039

use strict;
use warnings;

my $Pipe = '';

if( -p STDIN ) {
    while( <STDIN> ) {
        $Pipe .= $_;
    }
}

print "This was read from the pipe:\n";
print "<$Pipe>\n\n";

print "This was the read from the parameters:\n";
print "<@{ARGV}>\n";
__END__
[download]

Will prevent that hang:

C:\test>ReadPipeAndArgs.pl  A B CDE H 123
This was read from the pipe:
<>

This was the read from the parameters:
<A B CDE H 123>
[download]

But it prevents supplying the data by stdin redirection rather than a pipe:

C:\test>ReadPipeAndArgs.pl  A B CDE H 123 < ReadPipeAndArgs.pl
This was read from the pipe:
<>

This was the read from the parameters:
<A B CDE H 123>
[download]

Which would work without the pipe test.

So, perhaps a better way would be to only read from STDIN, if it is not connected to a terminal, which allows both modes of input and prevents the hang if neither is supplied:

C:\test>ReadPipeAndArgs.pl  A B CDE H 123 < ReadPipeAndArgs.pl
This was read from the pipe:
<#!/usr/bin/perl

# http://perlmonks.org/?node_id=1176039

use strict;
use warnings;

my $Pipe = '';

unless( -t STDIN ) {
    while( <STDIN> ) {
        $Pipe .= $_;
    }
}

print "This was read from the pipe:\n";
print "<$Pipe>\n\n";

print "This was the read from the parameters:\n";
print "<@{ARGV}>\n";
__END__
>

This was the read from the parameters:
<A B CDE H 123>

C:\test>type ReadPipeAndArgs.pl | ReadPipeAndArgs.pl  A B CDE H 123
This was read from the pipe:
<#!/usr/bin/perl

# http://perlmonks.org/?node_id=1176039

use strict;
use warnings;

my $Pipe = '';

unless( -t STDIN ) {
    while( <STDIN> ) {
        $Pipe .= $_;
    }
}

print "This was read from the pipe:\n";
print "<$Pipe>\n\n";

print "This was the read from the parameters:\n";
print "<@{ARGV}>\n";
__END__
>

This was the read from the parameters:
<A B CDE H 123>

C:\test>ReadPipeAndArgs.pl  A B CDE H 123
This was read from the pipe:
<>

This was the read from the parameters:
<A B CDE H 123>
[download]

That's not an empty pipe, just a pipe that has not yet produced a value.