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'
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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.