# code not tested!
my $srcFile = "/path/to/file.ext";

open F, "<", $srcFile; # this trip is READ
my @data = <F>;  # this can be danger if you have a big file
close F;

open F, ">", $srcFile; # this trip is WRITE
foreach my $line ( @data ) {
  $line =~ s/your_regex/expected_replacement/;
  print F $line;
}
close F;