Please tidy up that response, it is badly formatted.

$ ( echo Hi; sleep 3; echo Bye ) | ./ReadAPipe.pl a b c
This was read from the pipe:
<Hi
Bye>

This was the read from the parameters:



./ReadAPipe.pl a b c
This was read from the pipe:
<>

This was the read from the parameters:

Saving your (cleaned up) ReadPipe.pm to a sensible location and running your ReadAPipe.pl, I get:

qwurx [shmem] ~> ( echo Hi; sleep 3; echo Bye ) | ./ReadAPipe.pl a b c
This was read from the pipe:
<Hi
Bye>

This was the read from the parameters:
<a b c>
qwurx [shmem] ~> ./ReadAPipe.pl a b c
This was read from the pipe:
<>

This was the read from the parameters:
<a b c>
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I suspect that the code / result you posted isn't consistent (version mismatch?)

Please note that the value returned requireing your ReadPipe.pm (which is invoked by use) is the number of elements contained in @EXPORT_OK - which might, or might not, be intended. You should be aware of this side-effect. If your module read

...
our @EXPORT_OK   = qw( ReadPipe );
my $foo;
...
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then loading the module would fail, since the value computed from my $foo is undefined.