#!/usr/bin/perl
use CGI qw/:standard/;
print header;

$form_field = param("form_field"); # get the value from form
if ($form_field eq ""){            # if value is nothing
&print_error;                      # run subroutine
}else{                             # else
print "$form_field\n";             # do your stuff
}

sub print_error{                   # Error subroutine
print "there was an error\n";
}