Always check and report the result of a system action like "open", etc. Always write something like :

open(OUT, ">plot.gif") or die "can't open! $!";
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or if you want your script to go on :

open(OUT, ">plot.gif") or warn "can't open! $!";
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Update your code this way and see if there's an error message there that would enlighten you (hint: there's almost certainly one).