The first thing I notice is that you match the text, but never use the results of your match. After the if statement, do something like this:

if ( m/^\.SUBCKT\s+($runset{SchCell})/ ){
  my $match = $1;
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Then $match will contain your matched string.

After declaring $word and assigning $_ to it, you never use its contents. $word is overwritten on each iteration of the for loop.

What is in @_, and why are you looping over it? Are you trying to get the words that matched earlier? If so, you should do something like this instead:

foreach my $word (split ' ', $match) {
  if ($word =~ /^V(?:DD|CC)\z/) {
    # push @PN, etc.
  }
  elsif ($word =~ /^(?:VSS|GND)\z/) {
    # push @GN, etc.
  }
}
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I hope is helpful in pointing you in the right direction.