The BEGIN block is executed at parse time, but only when the parser finds it - it works top-down so it finds the prototype definition after it has parsed the call, when it is too late.

There is an alternative, which uses a similar technique to C, and that is to declare the prototype before the call. For example:

use warnings;
use strict;

mysub('duff');   

sub mysub() {
        print "mysub\n"
}
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Gives:

main::mysub() called too early to check prototype
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But we add the declaration:

use warnings;
use strict;

sub mysub();    # mysub declaration

mysub('duff');

sub mysub() {
        print "mysub\n"
}
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and we get:

Too many arguments for main::mysub
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as expected.