VinsWorldcom@C:\Users\VinsWorldcom\tmp> perl -v

This is perl 5, version 16, subversion 1 (v5.16.1) built for MSWin32-x64-multi-thread
[...]

VinsWorldcom@C:\Users\VinsWorldcom\tmp> cat test.pl
my $foo = "stuff";
open FD, "file.txt";
@data = <FD>;
close FD;

foreach (@data) {
  s/XXYY/$foo/;
  print;
}

VinsWorldcom@C:\Users\VinsWorldcom\tmp> cat file.txt
XXYY

VinsWorldcom@C:\Users\VinsWorldcom\tmp> test.pl
stuff