Re-opening the same file over and over isn't very efficient, so why not read the file into memory and work on it there?. splice lets you remove a chunk of lines from an array, and <> lets you read a file into an array.

So, something along these lines should work.

use v5.18;
use warnings;
use autodie;


my @replace = (qw/vss vdd/);

open(my $in,'<','input.txt');
my @lines = <$in>;
close $in;

LOOP:
for my $word (@replace) {
  my $start = -1;
  for my $i (0..$#lines) {
    if ($start == -1) {
      next unless $lines[$i] =~ /Power\s+information\s+for\s+$word\s+/
+;
      $start = $i;
    } else {
      next unless $lines[$i] =~ /}/;
      splice @lines,$start,$i - $start + 1;
      redo LOOP;
    }
  }
}
say @lines;

# save @lines to temporary file name
# rename input.txt to backup.txt
# rename temporary file to input.txt
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