You've received a working answer, now for a little explanation. With the code above, you ended up with $mdl = 1 and $page has its contents clobbered (i.e., changed to "page_name.pl"). Substitution (s///) returns the number of successful matches that occurred, and that's what you (inadvertently) said you wanted.
If you change the code to (my $mdl = $page) =~ s/\.tmpl$/\.pl/;, then the first thing that happens is that $mdl is set to the value of $page, and because it is the left-hand side of the assignment, $mdl then becomes the target of the =~ binding operation. Note the '$' anchor borisz and I added to the regex - this helps ensure you only change what you intend. What if $page = "my.tmplate.tmpl"?