Rather than have people guess, please be specific and tell us what modules you're using to do this. If you're using Imager then the documentation and warning are correct, a new image is returned on successful transformation:

my $newimg = $src->scale(xpixels=>400);
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From the docs:

"Returns a new scaled image on success. The source image is not modified. Returns false on failure, check the errstr() method for the reason for failure. A mandatory warning is produced if scale() is called in void context. "

Please read How do I post a question effectively?.