You can use the -p file test to check whether a filehandle is a pipe e.g

open(my $handle, "|date") or die("ack - $!");
print '$handle is a pipe', $/ if -p $handle;
print while <$handle>;

__output__

$h is a pipe
Fri Jun 21 17:10:26 GMT 2002
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But AFAIK you can't tell if STDIN has a pipe attached to the other end of it e.g

echo "foo bar baz" | perl -ne 'print uc'
# is the same as (in terms of content)
perl -ne 'print uc'
foo bar baz
^D
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Which is why the - option is common in many *nix command-line tools.
HTH

_________
broquaint