I can reproduce it with the following code, which works in 5.16.x and broke in 5.18.0: 
my $pat = qr/a/;
'aaaa'  =~ /$pat/gp or die;
print "MATCH=[${^MATCH}]\n";

[download]
Given that it was probably me who broke it, looks like I'll have to get it fixed for 5.18.1.

Dave.

In reply to Re: Upgrade to 5.18 causes uninitialized ${^MATCH} by dave_the_m
in thread Upgrade to 5.18 causes uninitialized ${^MATCH} by nbtrap

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