It looks fine to me.

$regel = "Username; abcd; Username"; 
$regel =~ s/Username/\nGebruikersnaam/sgm; 
print "(BEGIN)$regel(END)\n";
__END__
output:
(BEGIN)                 # <-- \n here
Gebruikersnaam; abcd;   # <-- and \n here 
Gebruikersnaam(END)
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Maybe the problem is in your output routine. Are you producing HTML? If so, a "\n" doesn't have any effect, unless inside <pre> </pre> tags.