if (/(\$[^\/\n\t]*)/) {
[download]

What if the presumed variable is $f.`rm -rf $ENV{HOME}`? Oops. Lotsa space now..

    my $var = $&;
    my $key = $var;
    substr($key, 0, 1) = "";
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You capture your match with () and then use $&? And what do you have $var for? Why not $key = $1?

Leave the the $ out of your capturing parens. That way you haven't got to strip it with substr.

I'd write your loop as

while(<FILE>) {
  if (my ($key) = /\$(\w+)/) {
    defined($ENV{$key}) and s/\$$key/$ENV{$key}/ or $undefinedVars++;
  }
  print;
}
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but it's usefulness depends on the input file...

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}