If I use "my $t" within this sub and localise it, we wont be referring to the first $t defined outside the sub.Is that right?
Yes - that's right. I thought maybe you were using $t for some other purpose, but it would appear not.

I'm not sure where we should use "my", and where we should not
Check out the section on 'scope' in whatever perl books/docs you have. 'my' declares a variable within the 'current scope' - if that's in a sub (or any pair of braces),it's gone when you exit the sub. If, on the other hand, it's in the 'main body' of your code, it's global oops - I mean file scoped...ermm..global to your file...ermm...you know what I mean - see the AM posting below... . That is generally a Bad Thing.

Turning on use strict gives no message about $t.
Does it give messages about other things? What about use warnings? Fix these, and your problem may possibly go away.

I haven't any more specific ideas, but here are a few general suggestions.

• use strict; use warnings;
• Don't use globals if you can help it. Pass arguments to subroutines instead - this helps your code become more 'general'
• use strict; use warnings;
• Refactor and tidy up. sub replace1,sub replace2 etc. look like they should be candidates for a single sub, with the replace string (and probably the text widget as well) passed in as a parameter.
• use strict; use warnings;
• Debug. Lots. Print out the values of $t after you initialise it, before you use it, after you use it etc. Check they're all the same type/memory address.