sub len
{
    length;
}

my @array = qw( fu bar baaaaz );

foreach ( @array )
{
    print len;
}
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If that's all you want, there you go. Beware of a couple of things though. First, you'll need to make sure that Perl knows what len means -- either declare the sub before, as I have done, or use a forward declaration. Second, some built-ins have special prototypes. You can use prototype to discover them. There are a couple of gotchas I always have to look up documented there.