It looks like you set $page to "" and then possibly set it to undef in the following line if there is no query parameter named "page". By the time perl compares $page to 'main', it's getting an undef value.

You probably meant something like: my $page = $query->param("page") // '';

or: my $page = $query->param("page") || '';

or: my $page = $query->param("page") || '';

Caveat: That will also assign '' to $page if $query->param("page") returns 0 or "0" (the same applies to hippo's answer).

TIMTOWTDI: Another alternative to the code from the OP may be (at least on Perl v5.12 and up):

sub loadPage {
    my $page = $query->param("page");    
    if ( length($page) && $page eq 'main' ) {
        loadMainPage();
        return 1;
    }
}
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    my $page = "";
    $page = $query->param("page");
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Don't do that if you want to test $page afterwards. Instead do something like this:

    my $page = $query->param ('page') || '';
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This way $page will always be defined, even if you don't have a param with the name "page".

providing a Short, Self-Contained, Correct Example would help, since there are obviously not 41 lines in your snippet. I am assuming "line 41" is the if ($page eq 'main') { line. In which case, the warning is there because $page is not defined. That would happen if, for example, you were not given a "page" parameter when the CGI script was run (ie, the URL didn't include page=blah, or you didn't pass that on the command-line when debugging your CGI script).

Hi guys, First thx for the interest you are been taking, here's a full cgi sample I've rewrite to bbe simple

#!/usr/bin/perl -w

use strict;
use warnings;
use CGI;

my $query = CGI->new ;
loadPage ();



sub loadPage {
    my $page = "";
    $page = $query->param("page");    
    if ($page eq 'main') {
    print "Content-Type: text/html\n\n";
        print "main page to load";
    }else {
        print "Content-Type: text/html\n\n";
        print "error";
    }
}
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this code work well now, I'm searching on other modules to see what's wrong thx Alexandre Jaquet

Almost, you pass arguments to subroutines, not work on globals. see cgi101/Re: Object Identifier? (red flags, more subs)

#!/usr/bin/perl --
use strict;
use warnings;
use CGI;
Main( @ARGV );
exit( 0 );
sub Main {
    my $q = CGI->new;
    my( $headers, $body ) = loadPage( $q );
    print $headers, $body;
}
sub loadPage {
    my( $query ) = @_;
    if( authenticate( $q ) ){
        my $page = $q->page || 'default';
        return ThisPage( $pms, $q  ) if $page eq 'thisone';
        return ThatPage( $pms, $q  ) if $page eq 'thatone';
        return DefaultPage( $pms, $q );
    } else {
        return UnauthorizedPage( $pqms, $q );
    }
}
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