#!/usr/bin/perl

use warnings;
use strict;

open PARAMS, "params.txt" or die "Couldn't open the file. $!";
my @params=<PARAMS>;
close PARAMS;

my @files = glob("*.RPT");
for my $i (@files) {
  open INPUT, "$i" or die "Couldn't open the file. $!";
  open OUTPUT, ">$i.dat" or die "Couldn't write to $i.dat - $!";

  print OUTPUT "$i\n\n";
  print OUTPUT " Parameter         Value\n";
  print OUTPUT "-------------------------\n";
  while (<INPUT>) {
    for my $x (@params) {
      next unless /\s($x)\s*(\d*)/;
      print OUTPUT "$1 $2\n";           # probably use printf() here
    }
  }
  close INPUT;
  close OUTPUT;
}

##</code><code>##

...
my $paramre = join '|', @params;
...
while (<INPUT>) {
  next unless /\s($paramre)\s*(\d*)/;
  print OUTPUT "$1 $2\n";
}
...