A demo:

#!/usr/bin/perl

eval 'undef %x::; sub yak {$x::a}; $x::a = 1';
eval 'print "t1: ",keys %x::,"\n"';
eval 'print "t2: ",keys %x::,"\n"; $x::a = 2';
eval 'print "t3: ",yak,",",$x::a,"\n"';
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This outputs the following:

t1: 
t2: a
t3: 1,2
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The first eval creates a glob *x::a in the symbol table, compiles both of its own $x::a accesses as references to that glob, then removes it from the symbol table upon executing the undef statement. As a result, there are no entries remaining in the symbol table, so t1 shows nothing. The third eval (t2) creates another glob named *x::a in the symbol table, prints the symbol table (showing *x::a still inside), and assigns the new $x::a a value. The fourth eval (t3) compiles its $x::a as a reference to the one currently in the symbol table, then accesses the original $x::a through a subroutine which was compiled with a reference to it, and prints both. (yak could just as well have been defined before any of the evals, since all that matters is that its compilation occurred before the undef was executed.)