$ 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