#!/usr/bin/perl -w
use strict;

# This sequence works, although with a warning...
# because Perl hasn't yet seen subroutine X.

X("xyz");

sub X()  # this means that subroutine X cannot 
         # be called with any arugment at all.
         # sub X();      #is ok, 
         # sub X("abc"); #is not ok.
{
   my $input = shift;
   print "$input\n";
}

#this would fail to produce a result - program fails to compile
# X("abc");  
# because now that subroutine X() has been seen, it is understood
# that no arguments can be passed to it.

__END__
prints:
main::X() called too early to check prototype at C:\TEMP\prototypes.pl line 4.
xyz