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"
}
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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