Start with use strict; use warnings; and clean up the mess that shows ya.

I suspect $F[$n] should be $Fn, ditto for $G[$n] to $Gn

its 

 
if ($flag eq 'C_N' || $flag eq 'C_S' || $flag eq 'C_E' || $flag eq 'C_
+W'){

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not 
if ($flag eq 'C_N', 'C_S', 'C_E', 'C_W'){

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you say 

  for my $i (0..$N) {
    for my $j (0..$N) {

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but your init statements only do 
 
for $i (1..$imax) {                
  for $j (1..$jmax) {

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a numeric equals test is == not =, the single equals sign is an assignment

after that i got it to compile, dunno if it does anything

In reply to Re^5: Particle movement question by huck
in thread Particle movement question by koda123

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