Don't you simply want open(FILE, $file)?

$ 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
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If you want STDIN to be the default, prepend @ARGV = '-' if !@ARGV;.

$ echo foo | perl -e'@ARGV = "-" if !@ARGV; open FILE, $ARGV[0] or die
+ $!; print <FILE>'
foo
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Perl does all of this for you when you use <ARGV> or its shorter form <>.

$ 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
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Is there any possible way of equating file handles?

Equating? Sounds like comparing, but I think you mean assigning. If so, yes. FILE is short for *FILE. If you use the latter form, you'll have no problem.

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