It appears to depend on qr. 
my $pat = qr/a/p;
'aaaa'  =~ /$pat/g or die;
print "MATCH=[${^MATCH}]\n";

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Appears to work OK on 5.18.0.

In reply to Re^2: Upgrade to 5.18 causes uninitialized ${^MATCH} by cdarke
in thread Upgrade to 5.18 causes uninitialized ${^MATCH} by nbtrap

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