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

sub a {
  print "Hi from a\n";
}

sub b {
  print "Hi from b\n";
}

doit('a');
doit('b');
$main::c=1;
$main::c++;
doit('c');

sub doit {
  my $func=shift;
  unless (exists($::{$func}) && defined(&{$::{$func}})) {    
    print "Invalid function called: $func\n";
    return 1;
  };
  no strict 'refs';
  &{$func};
}
__END__
Hi from a
Hi from b
Invalid function called: c