I guess there are alternatives, but it works:

# can't be called with a literal string as arg
# but that wasn't asked for :)
sub appendMissingSlash {
    return $_[0] .= substr($_[0], -1) eq "/" ? "" : "/";
}

my $string = "/tmp/check";
appendMissingSlash($string);
print $string,"\n";
__END__
/tmp/check/
[download]

Update: I didn't use a regex, as other examples given suggest, because we know the position and the character to check for, using the regex engine seemed overkill to me