It is during the parsing/compiling stage that it is complaining as it hasn't seen the sub declaration when it sees foo;. You can fix this by calling it as you have said above with foo(); or by predeclaring before you use it. 
use strict;
...
sub foo;
...
foo;
...
...
sub foo
 { ... }

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In reply to Re: Forward-referenceing subs by Paladin
in thread Forward-referenceing subs by John M. Dlugosz

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