$fh =~ s/\s//g;
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You can not apply a substitution to a file handle. (Well you can, but it does not modify the actual file content.)

Another problem is:

open(my $fh, ">> uuu.txt")
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...which would open the file for appending, but NOT for reading.

You did not specify whether you want an in-place edit of the file or whether you just want to convert the content for further processing. (For an in-place edit I would actually prefer to use something like 'sed -i~ s/[0-9 ]//g uuu.txt' instead of perl, because I get a backup copy of the original file for free.)