As documented in Variables related to regular expressions and lower down for $1:
These variables are read-only and dynamically-scoped.
The dynamic scoping (see also Temporary Values via local()) makes sense because it means code like this still works:
sub do_something_else {
my $foo = "quzbaz";
$foo =~ /([aeiou]+)/
and print ">$1\n"; # prints ">u"
}
my $bar = "foobar";
if ( $bar =~ /([aeiou]+)/ ) {
do_something_else();
print "$1\n"; # still prints "oo", not "u"
}
As a general rule, regular expression variables that you want to keep for later use should be copied into other variables ASAP, and only if the match was successful.
Update: Better link instead of local
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