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$ echo foo > foo.txt

...

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

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

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

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

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