I'd write it like this:

use strict;

open my $in, '<', "G203.xml" || die "Cannot open file: $!\n";
open my $out,'>' "_G203.xml" || die "Cannot open file: $!\n";

while (my $string1 = <$in>) {
  $string1 =~ s/XXXGASPOINTIDXXX/656563/gi;
  $string1 =~ s/XXXMMREFXXX/62394732472856563/gi;
  print $out $string1;
}

close $in;
close $out;
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Notes:

• I use the 3 argument form of 'open'.
• I read from the file one line at a time, using 'while', process each line, then write it out. No need to hold all the lines in memory (in an array).
• There's no need to sleep.
• There's no need to loop over the entire file twice: you can do both regex's inside a single loop. Two passes might be necessary if the patterns somehow overlapped, but these don't seem to do so).