See line 5811 of toke.c (for Perl5.6.0):

else if (*s == '\'' && isIDFIRST_lazy_if(s+1,UTF)) {
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So, in effect, /\$'[a-zA-Z_]/ is treated differently than /\$'[^a-zA-Z_]/ [note the caret (^)].

        - tye (but my friends call me "Tye")