my $filename = "C:\\temp\\testdata.txt";
open ( FILE, $filename) or die "Cannot open file: $!";

while ( $line = <FILE> ) {
    # i is case insensative
    # ([^>]*) match zero or more characters but not '>'
    $line =~ s/ying/yang/gi;
    push(@outLines, $line);
}

close FILE;

open ( OUTFILE, ">$filename" );
print ( OUTFILE @outLines );
close ( OUTFILE );

?>