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It's possible for either or both of pos or  ${^MATCH} to be undefined after a match, which is easily demonstrated (Perl version 5.14):

>perl -wMstrict -le "my @strs = (qw(123x foo ***), ''); ;; for my $s (@strs) { $s =~ m{ \d+ }xmsgp; printf qq{for '%s': pos == %s, \${^MATCH} eq '%s' \n}, $s, pos($s) // 'undefined', ${^MATCH} // 'undefined', ; } " for '123x': pos == 3, ${^MATCH} eq '123' for 'foo': pos == undefined, ${^MATCH} eq 'undefined' for '***': pos == undefined, ${^MATCH} eq 'undefined' for '': pos == undefined, ${^MATCH} eq 'undefined'

Rather than Perl, I think the most likely explanation is that the data you are processing has changed.

Update: ... and the length of an undef is undefined.

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

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