If you want them separate then remove the g modifier.

$_="a1 a2 a3";
while(?a(\d)?){ print $1; }
while(?a(\d)?){ print $1; }
__END__
11

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Updated to add in the snippet.

In reply to Re^3: Bizarreness in ?PATTERN? and g by Mr. Muskrat
in thread Bizarreness in ?PATTERN? and g by BUU

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