I don't know how perl pre-parses code on the fly - is it quicker (i.e. will perl map out a call to a sub-routine on pre-parse) such as

if ($debug == "true") {
&debugit;  # call the routine
}
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or

if ($debug == "true") {
** do all the debug code here and/or carry on **
}
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Premature optimization. Keep the code readable and maintainable. If you need the debug code more than once, put it in a function. If the debug code spans more than a few lines, do the same. For a single, simple print, wrap it directly in the if block.

Oh, and by the way: Omit the ampersand in function calls. It is perl4-style and does not do what you expect. See also Re^2: Merge log files causing Out of Memory (just a note on ampersand).

Another update: $debug == "true" does not do what you want. == is for numeric compares, eq is for string compares. (how to remember that). But you don't want either, use Perl's native booleans: In boolean context, scalars that are undefined (undef), the empty string ("") or the number zero (0) evaluate as false, any other value evaluates as false true. Commonly, you would use the value 1. So:

my $debug=1; # or $debug='' to disable
# later:
if ($debug) {
    debugIt();
}
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Alexander