gpl and web scripting

[Moritz Sinn]

hi,

i've a question: lets say i write some software (e.g. with perl) that
works web based (all you need to use that software is a proper
webbrowser).
now lets say i publish it under GPL. If now someone downloads the code
and changes it and then puts it on his webserver so that everyone can
use it there, does he have to publish the changes under GPL?
I mean, its clear that if someone changes it and then pass the changed
program on, he has to publish that version under GPL. but if he doesn't
pass it on, if he just makes it useable, but it runs on his own
webserver, is that the same?
All he does is modifying the code and running that version on his
public webserver.

Would be nice if someone could explain me why he has to publish it under
GPL or why not.

regards,
 moritz

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