use warnings;
use strict;

eval ' package T; print "A: $x\n"; 1 ' or warn "A: $@";
# => Global symbol "$x" requires explicit package name

{ no warnings 'once'; *T::x = \"foo"; } # runtime of main!

eval ' package T; print "B: $x\n"; 1 ' or warn "B: $@";
# => now works