why do you need vim for stdin and stdout? just use:

$a = <STDIN>; # This is one way of how you read user input
print $a, "\n";  # This is one way of printing to stdout
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or if you want achieve something like this cat some_file | some_perlscript.pl, then you could write something like this in your script:

while(my $line = <>) { # reads from stdin
   print $line; # prints everything to stdout
}
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but if you really want to start vim from a perl script, just use the backticks:

`vim $some_file`;
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maybe if you gave us some more specs, we'd understand you better ;-)