Some of the answers here seem to be missing the point - or maybe it is me missing the point? The transformation you describe on your code is, as I understand it,

sub Main {
    local ($list_of_vars);
    
    [old code, which thinks the above locals are globals]
}
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Yes? In which case, the 'global' variables (global to the old code, but not _global_ :) are only instantiated _once_, throughout the lifetime of the program. So, unless the main amount of work you do is defining a load of variables (highly unlikely), you won't gain anything switching local to my.

To be honest, if you want faster scripts with minimal optimization (i.e., no algorithm analysis, etc.) just run it on a faster machine. A lot of people will baulk at that, but look at the costs involved and make the decision.

Cheers.

You typically do a lot of work in accessing variables. Lexical variables bind at compile-time, global variables involve a run-time lookup. Hence the performance difference.

Personally I suspect that with that much code, all global, there are serious algorithm mistakes scattered around that cannot be seen let alone fixed because of the hole that they dug for themselves...

Exactly. :)