$ echo foo > foo.txt

$ perl -e'open FILE, $ARGV[0] or die $!; print <FILE>' foo.txt
foo

$ echo foo | perl -e'open FILE, $ARGV[0] or die $!; print <FILE>' -
foo

$ perl -e'open FILE, $ARGV[0] or die $!; print <FILE>' 'echo foo |'
foo

$ perl -e'open FILE, $ARGV[0] or die $!; print <FILE>' 'cat foo.txt |'
foo

##</code><code>##

$ echo foo | perl -e'@ARGV = "-" if !@ARGV; open FILE, $ARGV[0] or die $!; print <FILE>'
foo

##</code><code>##

$ perl -e'print <>' foo.txt
foo

$ echo foo | perl -e'print <>' -
foo

$ echo foo | perl -e'print <>'
foo

$ perl -e'print <>' 'echo foo |'
foo

$ perl -e'print <>' 'cat foo.txt |'
foo

##</code><code>##

*FILE = *STDIN;
$fh   = *STDIN;
$fh   = \*STDIN;    # Ref often allowed and even preferred.