I haven't looked at the source for Unicode::UCD, but I'm guessing that it's probably doing some file input somewhere. (Not an unreasonable thing.)

That's one good reason why slurp mode should really be done like this, especially in code that uses modules, to make sure that undef'ing $/ is localized to just the code block where this is needed:

my $filedata;
open( IN, '<', $filename ) or die "whatever. $!";
{
    local $/;
    $filedata = <IN>;
}
close IN;
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