This will help you. I always use this technique.

my $str = '<a code=TXT> abc TXT </a> <text code=TXT> efg TXT </text> <
+input s=TXT> hig TXT</input>';
print "Before: ",$str,"\n#########\n";
$str =~ s{(>)(.*?)(<)}
{
  my ($tag_ends, $mydata, $tag_starts) = ($1,$2,$3);
  $mydata =~ s/TXT/FOO/gs;
  "$tag_ends$mydata$tag_starts"
}exgs;
print "After: ",$str,"\n#########\n";
[download]

Let me know if you need any more help.