Note that from 5.9.0 onwards you get a run-time warning:

$ perl590 -w /tmp/p
Variable "$x" is not available at (eval 1) line 1.
Use of uninitialized value in concatenation (.) or string at (eval 1) 
+line 1.
x=
$ cat /tmp/p
#!/usr/bin/perl

{
    my $x = 1;
    sub f { eval q(print "x=$x\n") }
}
f;
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Dave.