my $package = 'main';
my $function = 'foo';

{
    no strict 'refs';
    if (defined &{$package.'::'$function}) {
        print "$function is defined in $package\n";
    }
}
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We are the carpenters and bricklayers of the Information Age.

Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.