open (INFILE, "input.txt") or die "cannot open input: $!";
open (OUTFILE, ">output.txt") or die "cannot open output: $!";

while (<INFILE>) {   # note the < and > brackets

   s/foo/bar/sg;     # no =~ needed for $_
                     # no m needed

   print OUTFILE;    # no "$_\n" needed: $_ is automatic
                     # and the \n is still in $_ (you
                     # don't cho(m)p it...
      
}

close INFILE;
close OUTFILE;