You can check if a key exists or is defined without creating it. You could insert that into your test like so:

if ( exists $CATEGORY_DATA{$FD_NAME}
     && $FD_NAME eq $CATEGORY_DATA{$FD_NAME}{'FIELD_NAME'} )
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You could also only loop over the keys of your hash in the first place.

foreach my $FD_NAME ( keys %CATEGORY_DATA )
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