For the "too early to prototype" message, you need to have the function definition (or at least declaration) before the call. Compare:

#!/usr/bin/perl -w

use strict;

check_for_in( "derby" );

sub check_for_in( $ )
{
   my( $arg ) = shift;
   print  $arg, "\n";
}
[download]

to

#!/usr/bin/perl -w

use strict;

sub check_for_in( $ )
{
   my( $arg ) = shift;
   print  $arg, "\n";
}

check_for_in( "derby" );
[download]

-derby