[perl #73712] ‘Variable is not imported’ cannot be suppressed

[perl #73712] ‘Variable is not imported’ cannot be suppressed

The message ‘Variable "%s" is not imported’ cannot be suppressed, even
with -X (local $SIG{__WARN__}=sub{} is what I have to use):

perl -Xle '$foo;use strict; eval q/$foo/ or die "---$@---"'
Variable "$foo" is not imported at (eval 1) line 2.
---Global symbol "$foo" requires explicit package name at (eval 1) line 2.
--- at -e line 1.

This is because we have what appears to the user to be a multi-line
error message. It is in fact a warning ‘Variable...’ followed by an
error ‘Global symbol...’.

The attached patch assigns a warning category to the warning.