I wouldn't use a regular expression, but something like this, probably:

sub before_nth_char
{
    my ($str, $c, $n) = @_;

    my $pos = -1;

    {
        $pos = index $str, $c, $pos + 1;
        return if $pos == -1;
        redo unless --$n == 0;
        return substr $str, 0, $pos;
    }
}

print before_nth_char("asdfasdfasdf", "a", 2), $/;
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