if (*package::foo{CODE}) {
    # Function package::foo exists.
} else {
    # Function package::foo does not exist.
}

##</code><code>##

my $function = "package::foo";
{
    no strict qw/refs/;   # We're going to use symbolic refs.
    if (*{$function}{CODE}) {
        # Function in $fuction exists.
    } else {
        # Function in $function does not exist.
    }
}