You don't need to set $1 before the match. In fact, I think you can't. 

$ perl -le '$1=undef;'
Modification of a read-only value attempted at -e line 1.

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$1 will be set to undef after a match with no parentheses. 

"1234"=~/(\d)/;
print "$1\n";
"1234"=~/34/;
print "$1\n";
print "undef\n" if ! defined $1;

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Prints: 

1

undef

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This doesn't work when a regex had capturing parentheses that didn't match. Just like the "no parentheses" case above, this also prints "undef": 

"1234"=~/(x)?34/;
print "undef" if ! defined $1;

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In reply to Re: Determing whether a regexp has capturing matches by kyle
in thread Determing whether a regexp has capturing matches by adrianh

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