You can combine calin's glob assignment with local to get dynamic scoping of such constants,

{
    local *foo = \"foo";
    bar()
}
sub bar {
    print "${foo}bar$/"
}
bar();
__END__
[download]

which prints,

foobar
bar
[download]

unless you have warnings on, which will complain of undefined $foo in the second call of sub bar.