This also happens with code one of my friends is maintaining.
On his machine (NT?), indexing an array beyond its 'top' element
extends the array, and so a loop (which used to run fine) never
terminates:

my @main = ( "<UL>\n", "<!--begin-->\n", "</UL>\n" );

&getboardinfo();

sub getboardinfo
{
    my($tnum);
    
    $msgcount = 0;
    $lastmsg  = 0;
    $lastfu   = 0;
    $fucount  = 0;
    
    print STDERR "Going into loop: main is @main\n";
    print STDERR "main has ", $#main + 1, " elements.\n";
    
    for( $j=0; $j<=@main; $j++ ) 
    {
        print STDERR "Loop! j is $j main[j] is $main[$j]  main[j+2] is
+ $main[$j+2]\n";
[download]

An illustration of the problem lies in the code above.
The reference to $main[$j+2] extends the array, and so
the loop goes on forever.

        
        if( $main[$j] =~ /<!--top: (.*)-->/ ) 
        {
            $msgcount++;
            $lastmsg = $1;
        }
        elsif( $main[$j] =~ /<!--ins: (.*)-->/ )
        {
            $tnum=$1;
            
            if( ( $main[$j+1] =~ /<!--end:/ )
              &&( $main[$j+2] =~ /<!--end:/ ) )
            {
                $lastfu = $tnum;
                $fucount++;
            }
        }
    }
    print STDERR "Done with loop\n";
    return 0;
}
[download]

Of course, the original writer should probably have found
a better way to write this loop (maybe by using foreach()).

Rob